Magnetically tunable band-stop and band-pass filters



Aug. 23, 1966 MAGNETI CALLY G. L. MATTHAE! 3,268,838

'I'UNABLE BAND-STOP AND BAND PASS-FILTERS Filed May 20, 1964 FIG! H FERRIMAGNETIC MATERIAL FIG. 4 H

30A FERRIMAGNETIC MATER1AL 26 T 27 25 2| 23 x 1 27 f 25 INVENTOR,

V GEORGE L. MATTHAEI.

l 26 BY M 232M 30 M W 1%! M4! M49444,

ATTORNEY;

United States Patent MAGNETICALLY TUNABLE BAND-T6]? AND BAND-PASS FILTERS George L. Matthaei, Menlo Park, Callf., assignor to the United tates of America as represented by the Secretary of the Army Filed May 20, 1964, Ser. No. 369,026 8 Claims. ((Il. 33373) The present invention relates to magnetically tunable filters and more particularly to magnetically tunable bandstop and bandpass filters which utilize ferrimagnetic resonators.

Magnetically tunable filters are of interest for applications Where it is desired to blank out or pass a desired signal, using electronic control to adjust the frequency. For example, one possible application for a magnetically tunable band-stop filter is in combination with a sweptfrequency superheterodyne receiver to eliminate the image response. As the receiver operating band is swept, the stop band of the filter could also be swept so as to continuously eliminate the image response.

The resonators in the filters of the present invention use gyromagnetic resonance in spheres of such material as single-crystal, yttrium-iron garnet (YIG). In YlG resonators, the resonant frequency can be controlled by varying a biasing D.-C. magnetic field. However, because of non-uniformities in the D.-C. or RF fields Within the resonators, which result in higher-order magnetostaticmode resonances, small spurious responses occur. These spurious responses become greater, causing unwanted hands, if the resonators are placed too close to metallic material. Therefore, coupling energy to the resonators cannot be increased past a definite limit by merely placing the resonators closer to the filter structure without causing unwanted spurious responses. The present invention provides a filter structure which provides increased coupling to the resonators without requiring the resonators to be placed closer to the filter structure. Besides increasing the coupling the present invention results in a structure which is relatively reduced in size.

It is therefore an object of the present invention to provide a means for increasing the coupling to gyromagnetic resonators without causing unwanted spurious responses.

It is another object of this invention to provide magnetically tunable filters requiring magnetic pole faces which are relatively small.

The exact nature of this invention as well as other objects and advantages thereof will be readily apparent from consideration of the following specification relating to the annexed drawings, in which FIG. 1 shows one form of the invention in an isometric sectional view taken on the line 11 of FIG. 2;

FIG. 2 shows a sectional end view of the device taken on the line 22 of FIG. 1;

FIG. 3 shows an isometric view, with parts broken away, of another form of the invention; md

FIG. 4 shows a sectional view of the device shown in FIG. 3.

With reference to the drawings wherein like reference numerals designate like parts throughout, there is shown in FIGS. 1 and 2 a strip line having two outer conductors 11 and 12 and a center conductor 13. Spaced along the strip line 10 are a plurality of shunt capacitors comprising metal capacitor blocks 14 connected to the outer and center conductor and separated by a small gap filled with a dielectric material 15. Mounted intermediate the capacitors and midway between the center conductor 13 and outer conductor 12 are ferrimagnetic resonators 16. These resonators may be spheres of such material 3,263,833 Patented August 23, 1966 "ice as single-crystal, yttrium-iron-garnet. The resonance characteristics of such materials are well known and will, therefore, not be discussed in detail. Generally, ferrimagnetic materials display resonance characteristics at a frequencywhich will be dependent upon the magnitude of a DC. magnetic field applied to the material. These materials will absorb energy at the resonant frequency which may be traveling along the strip line 10. The D.-C. magnetic field used for tuning the resonators 16 is represented by an arrow H which, of course, should be distributed uniformly over all of the resonators 16.

The exact operation of magnetically-tunable filters using ferrimagnetic crystals is discussed by P. S. Carter, Jr., in Magnetically Tunable Microwave Filters Using Single- Crystal Yttrium-Iron-Garnet Resonators, IRE Transactions, PGMTT9, pp. 252-260 (May 1961). In this article it is pointed out that the coupling of energy to the resonators is dependent upon the geometry of the strip line and the volume of the resonator sphere. For example, if the center conductor of a uniform strip line is brought closer to the outer conductor, coupling between the center conductor and the resonators will be increased. However, as was pointed out earlier if the resonators are placed too close to a metallic surface spurious responses are created. Therefore, the height of the strip line can be reduced only to a definite limit. Coupling may also be increased by notching or reducing the width of the center conductor in the vicinity of the resonators. This solution is not entirely satisfactory since the notched portion will reflect some of the energy and, if made too narrow, will cause fringing fields to be coupled to the resonators 16.

However, coupling energy to the resonators 16 can be increased as shown in the present invention by providing sections of high and low impedance and placing the resonators in the high impedance sections. Even though the resonator is located in the high impedance section, where the currents are concentrated, it will see the effective impedance of the entire structure which will be much lower. If the low impedance sections, provided by shunt capacitors 14, and the high impedance sections, provided by the center conductor 13 between the capacitors 14, are designed so as to make up an efiicient low pass filter which will pass all frequencies of interest, there will be a negligible amount of losses due to reflections. The resonators 16 may now be spaced a distance less than a quarter Wavelength apart, as is required in a uni-form strip line, because of these lumped capacitor and inductive sections. This structure will therefore reduce the size requirements of the filter and the magnet which will produce the field H. Coupling between resonators 16, which may cause unwanted modes, will be reduced because of shielding by the metal blocks 14. Coupling between resonators 16 may be reduced even further by placing alternate resonators 16 between the outer condoctor 11 and the center conductor 13. Of course, by increasing the coupling of energy from the strip line 10 to the resonators l6, wider stop bands and larger peak attenuation are obtained.

The above principles were tested by experiment. It was found that a particular YIG resonator coupled to a uniform 50-ohm transmission line gave a band-stop resonance with a peak attenuation of 12.5 db and a 3 db bandwidth of 9 mc. at 3000 me. The same sphere in a low pass filter structure similar to that in FIGS. 1 and 2 gave a peak attenuation of 21.3 db and a 3 db bandwidth of 26.7 me. at 3000 me.

These same principles may be applied to a band-pass filter such as shown in FIGS. 3 and 4 which show a pair of rectangular waveguides 20 and 21. Each waveguide 20 and 21 comprises a transformer section having reduced height steps 24 and terminates in a short circuiting end wall 22 and 23 respectively. In the section of the waveguides before the terminating walls 22 and 23 there is provided a section 25 of increased height and a section 27 of substantially reduced height. Mounted in section 25 is a spherical YIG resonator 26 on a mounting rod 30. An elongated slot 28 is provided in the wall 29 of each waveguide and 21 and in communication with the sections 25. These slots 28 are aligned with each other and with the resonators 26 and their long dimension is parallel to the axis of the waveguides 20 and 21.

The operation of the band pass filter may be described as follows:

Coupling betwen spheres 26 is obtained through the elongated slots 28 which have their long dimension parallel to the longitudinal axis of the waveguides 20 and 21. This orientation of the slots 28 causes minimum disturbance of the currents and fields in the waveguides 20 and 21, while furnishing maximum isolation between the waveguides 20 and 21 away from ferrimagnetic resonance. Thus, the coupling between the waveguides 20 and 21 is extremely small when the resonators 26 are not resonant. However, when the resonators 26 become resonant, the energy is absorbed by the spheres and the circularly polarized RF magnetic dipole moment in the spheres provides a component of RF H-field perpendicular to the input RF H-field and the DC. H-field, which will couple through the elongated slots 28 very easily. Thus, at resonance, energy coming in, for example, the waveguide 21, will be absorbed by YIG resonator 26 which will in turn generate an RF H-field which will be perpendicular to the DC. H-field and the input RF H-field. This generated RF H-field will be passed through slots 28 and be absorbed by the other YIG resonator 26 which will generate an RF H-field perpendicular to the field passing through slots 28 and the DC. field. This RF H-field generated by the second resonator 26 will now be oriented so as to be coupled to the waveguide 20. If the biasing magnetic field H is not adjusted to give resonance at the input frequency, the input signal will be reflected and no energy will pass through slots 28.

The waveguide height is reduced by a transformer section so as to increase the coupling to the resonators 26. Of course, the smaller the height the better will be the coupling to the resonators 26. However, as was stated before the height can be reduced to only a definite limit without causing spurious responses because of the metal walls. However, by providing a low pass filter structure having a short section 27 of substantially reduced height and a short section of increased height coupling energy to resonators 26 can be increased. Section 27 is the equivalent of a shunt capacitor having a relatively low impedance while section 25 is the equivalent of a series inductance of relatively high impedance. The image impedance which the resonator 26 sees will be somewhere in between the impedances of sections 25 and 27. This image impedance will represent a waveguide of a height less than the height of section 25 where the resonator is actually located. Therefore, as in the previous case of the strip line, the resonators 26 are coupled to a structure of relatively low impedance while actually being located in a section of relatively high impedance.

Of course, many other modifications and variations of the present invention are possible in the light of the above teachings. For example, these same principles may be applied to other types of transmission lines such as coaxial lines whether they be band-pass or band-stop filters. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than is specifically described.

What is claimed is:

1. A magnetically tunable band-stop filter comprising a transmission line for transmitting electromagnetic energy having at least first and second conductors, a plurality of capacitors mounted between said conductors and spaced along said transmission line, a plurality of ferrimagnetic resonators mounted between said conductors and spaced along said transmission line with one of said resonators being located between each of said capacitors, and mag netic means mounted adjacent said transmission line for tuning said resonators.

2. The filter according to claim 1 and wherein said transmission line comprises a parallel plane strip line.

3. The filter according to claim 2 wherein said strip line comprises a pair of parallel ground plane conductors and a center conductor mounted therebetween, and wherein said capacitors comprise metal blocks separated by a dielectric.

4. The filter according to claim 2 and wherein said resonators comprise spheres of single-crystal yttrium-irongarnet.

5. A magnetically tunable band-pass filter comprising first and second rectangular waveguides each said waveguide having a first section of reduced height followed by a second section of increased height and a terminating wall, a slot provided in the bottom wall of each said second section, the bottom walls of each said waveguide be ing mounted adjacent each other with said slots in align ment, ferrimagnetic resonator means mounted in each said second section and adjacent said slots and magnetic means mounted adjacent said waveguide for tuning said resonators.

6. The filter according to claim 5 and wherein each said resonator comprises a sphere of single crystal yttriumiron-garnet.

7. A magnetically tunable band-pass filter comprising first and second rectangular waveguides having transformer sections of decreasing height followed by a section of increased height and a terminating wall, the bottom wall of each said section of increased height having an elongated slot therein, the bottom walls of said waveguides being mounted adjacent each other with said slots aligned with each other, ferrimagnetic resonators mounted in said sections of increased height and aligned with said slots, and magnetic means providing a magnetic field across said resonators.

8. The filter according to claim 7 and wherein said resonators are spheres of single crystal yttrium-iron-garnet.

References Cited by the Examiner UNITED STATES PATENTS 2,877,433 3/1959 Devot 33373 3,001,154 9/1961 Reggia 33373 3,013,229 12/1961 De Grasse 33373 HERMAN KARL SAALBACH, Primary Examiner. P. L. GENSLER, Assistant Examiner. 

1. A MAGNETICALLY TUNABLE BAND-STOP FILTER COMPRISING A TRANSMISSION LINE FOR TRANSMITTING ELECTROMAGNETIC ENERGY HAVING AT LEAST FIRST AND SECOND CONDUCTORS, A PLURALITY OF CAPACITORS MOUNTED BETWEEN SAID CONDUCTORS AND SPACED ALONG SAID TRANSMISSION LINE, A PLURALITY OF FERRIMAGNETIC RESONATORS MOUNTED BETWEEN SAID CONDUCTORS AND SPACED ALONG SAID TRANSMISSION LINE WITH ONE OF SAID RESONATORS BEING LOCATED BETWEEN EACH OF SAID CAPACITORS, AND MAGNETIC MEANS MOUNTED ADJACENT SAID TRANSMISSION LINE FOR TUNING SAID RESONATORS. 